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83 Candidate HIV-1 Vaccines Induce Antibodies That Neutralize in Vitro Primary Isolates in Assays with Extended Incubation Phases
H. Donners*, D. Davis, B. Willems, T. Vermoesen, and G. van der Groen
Inst. of Tropical Med., Antwerp, Belgium
Background: So far, vaccine-induced antibodies have failed to neutralize significantly in vitro primary isolates, considered to be more refractory for neutralization. It is reasonable to assume that these isolates are more representative of those involved in natural transmission so that Phase III trials have been discouraged. However, the mechanisms of neutralization are not yet properly understood. We hypothesize that by modifying the design of a neutralization assay, 2 different neutralization-mechanisms can be monitored.
Methods: Neutralizing antibodies are quantified in assays using PHA-transformed PBMCs, GHOST cells, or dendritic, T4 lymphocyte cocultures as target cells. Different assay conditions are taken, referred to as x/y/z where: x is the time in hours for which antibody is incubated with virus; y is the time in hours allowed for virus to absorb to cells; z is the total culture period in days, from the cells’ first exposure to virus before antigen production (PBMC) or number of fluorescent cells (GHOST) are measured. Neutralization indices (NI) of >0.7 (80%) are considered as neutralizing.
Results: In vaccinated humans (24/24/2 GHOST assay), monkeys (24/2/14 dendritic/T4 coculture), or rabbits (24/24/14 PBMC assay), significant neutralization against primary isolates can be demonstrated in assays with extended incubation phases. In humans and monkeys 90 up to 99% reduction of infectivity are reported. In contrast, in some HIV-infected individuals, neutralizing antibodies can be detected against a wide range of primary isolates (group M [env A-H] and O) in 1/2/7 PBMC-assays. Neutralization can even be seen without preincubation (0/x/2 GHOST assay). In another HIV-infected individual, neutralizing antibodies can be detected against 8/16 primary isolates in 1/24/7 PBMC-assays. Although, when cultures are extended (1/24/14), only 1/16 is neutralized.
Conclusions: There are 2 different reactions occurring within an HIV-1 neutralization assay. In HIV-seropositive plasma, measuring a reduction of infectivity is independent of the incubation phase, whereas with vaccine-induced antibodies long incubation phases are required. If we wish to predict antibody-mediated protection in vaccine trials, it will require assays with extended incubation phases.
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